Metal-like properties, such as electrical conductivity, were first discovered in molecularly doped polyacetylene in 1977 (see Shirakawa et al., J. Chem. Soc. Chem. Commun., 1977, p. 578). Since that discovery, the characteristics of several electroactive polymers have been studied extensively.
The electrochemical polymerization of a monomer suitable for preparing an electroactive polymer was initially achieved utilizing pyrrole to form polypyrrole. It was determined that polypyrrole had an electrical conductivity of 100 Scm.sup.-1 and could undergo reversible oxidation and reduction by applying an electrical potential from about 0.8 volts to about -0.6 volts with respect to a saturated calomel electrode. Accompanying the oxidation/reduction process was an associated color change from blue-black to pale yellow, respectively.
The formation of electroactive polymer films on conductive substrates by electrochemical techniques has been found to produce polymer coated electrodes suitable for a variety of purposes arising either from the reversible oxidation and reduction of the polymer films or from the high electrical conductivity of the polymer films. As examples, electroactive polymer coated conductive substrates can be used to prepare electrochromic devices, "smart" windows, optical switches for information processing and charge coupled devices, electromagnetic interference devices, semipermeable membranes, catalytic electrodes, gas sensors, photovoltaic components, solid batteries, diodes, fast response non-linear optical materials, and electrostatic dissipation devices.
A severe limitation on the use of electroactive polymer coated conductive substrates for the purposes listed hereinabove, however, is the fact that electrochemically deposited conductive polymers generally have a non-uniform thickness and topography, are easily removed from conductive substrates by contact with a solvent or mechanically by moderate abrasion, and have widely ranging electrical conductivities from about 10.sup.-2 Scm.sup.-1 to about 10.sup.2 Scm.sup.-1. Thus, the non-uniformity and limited durability of electroactive polymer coated conductive substrates preclude their widespread use.
Miasic et al., "Electronically Conducting Polymer Gas Sensors," Conducting Polymers, D. Reidel Publishing Co., 1987, p. 189 discloses a method for depositing a film of polypyrrole directly onto a gold film by the electropolymerization of pyrrole from an aqueous solution, to produce an ambient temperature detection deivce for several industrial gases. The resistance of the polypyrrole film so produced increases in the presence of ammonia and decreases in the presence of hydrogen sulfide.
In Rubinstein et al., "Morphology Control In Electrochemically Grown Conducting Polymer Films. 1. Precoating The Metal Substrate With An Organic Monolayer," J. Am. Chem. Soc., 1990, 112, p. 6,135, a monolayer of individual p-aminothiophenol molecules was deposited onto the surface of a gold substrate to improve the adhesion thereto of an electrochemically grown polyaniline electroactive polymer film. The article states that the adhesion-promoting monolayer significantly increases the density of the electro-chemically grown polymer film, and results in a radiation absorption coefficient at 6,000 Angstroms about eight times higher than the average absorption coefficient for the same film grown on "bare" gold. The substantial increase in the electroactive polymer film density is attributed to the adhesion-promoting monolayer which facilitates and regulates the bonding between the modified gold substrate surface and the growing phase of polyaniline. It is stated that the beneficial effect obtained concerning film morphology is apparently caused by a more uniform and efficient nucleation-and-growth process on the treated surface, resulting in a film with significantly improved space filling. Thus, it is recognized that an adhesion-promoting layer between an electroactive polymer film and a conductive substrate provides the dual benefit of greater durability and increased polymer density and uniformity. The deposition of a monolayer of individual p-aminothiophenol molecules, however, is difficult to achieve and accurately control. It is felt that a monolayer or multiple molecular layer of a polymer, as opposed to a monolayer of individual molecules, would improve the integrity, adhesion, and density of a subsequently applied electroactive polymer film.
U.S. Pat. No. 4,468,291 to Naarmann et al. discloses a continuous process for forming a homogeneous, uniformly thick polypyrrole polymer or copolymer film. Pyrrole monomer, which may be mixed with other comonomers such as thiophene in ratios from 1:99 to 99:1, is added to an electrochemical cell containing a solvent and an electrolyte. The polypyrrole film is electrochemically polymerized onto a continuously moving anode which is immersed and moving through the electrolyte solution. The electropolymerization is carried out at a constant current density sufficiently high so as to electrodeposit a singular, homogeneous layer of the copolymer of pyrrole and the comonomer. The patent further discloses that a second layer of polypyrrole polymer or copolymer may be electrodeposited by the patented process onto a first layer of an electrically conductive polymer (which functions as the anode) such as polyacetylene or polyphenylene, thereby forming a two-layered polymer structure. Such a process may not be used, however, to deposit a first layer of polypyrrole and a second layer of the polymerized comonomer.
It would be desirable to prepare by a simple process an electroactive polymer coated conductive substrate, having improved polymer density, uniformity, and topography. Such an improved structure might be achieved by depositing a first layer of an electroactive polymer onto a conductive substrate, followed by the deposition thereover of a second electroactive polymer, wherein the first polymer layer functions as an initiator for the deposition of a second polymer layer having a uniform thickness and topography.